That's called dross and you're going to get some on your melt. Don't worry about it. It does have some bad stuff in it, so ladle it off the top of the melt and dispose of it. I normally keep a small can (think coke can with the top cut out.) beside the melter and when I add metal and flux, I skim off the dross and put it in the can. After three or four session, the can will get full and I throw it away.